The invention resides in the fields of molecular biology and medicine, and more particularly drug discovery.
The kinesin superfamily are comprised of proteins which utilize a conserved catalytic motor domain to generate intracellular movement of vesicles or macromolecules along microtubules in diverse eukaryotic cellular processes (e.g., cell proliferation). Over 90 kinesin proteins can be classified into at least 8 subfamilies based on primary amino acid sequence, domain structure, velocity of movement, and cellular function. The motor domain is a compact structure of approximately 340 amino acids, and can be located at the N-terminus, in the internal region, or at the C-terminus of the kinesin molecule. Most of the kinesin proteins have an N-terminal catalytic motor domains, e.g., the BimC and the KHC families (See, e.g., Goldstein et al., Annu. Rev. Cell Dev. Biol., 15:141-83,1999; Moore, J. D. and Endow, S. A., Bioassays 18:207-219, 1996). During mitosis, kinesins organize microtubules into the bipolar structure that is the mitotic spindle, mediate movement of chromosomes along spindle microtubules, as well as structural changes in the mitotic spindle associated with specific phases of mitosis. These xe2x80x9cmolecular motorsxe2x80x9d translate energy released by hydrolysis of ATP into mechanical force which drives the directional movement of cellular cargoes along microtubules.
The prototypical native kinesin molecule is a heterotetramer comprised of two heavy polypeptide chains (KHC""s) and two light polypeptide chains (KLC""s). The KHC homodimer is typically referred to as xe2x80x9ckinesinxe2x80x9d and is classified as a member of the KHC kinesin family (Goldstein et al., Annu. Rev. Cell Dev. Biol., 15:141-83,1999). The human form of KHC has been cloned (Navone et al., J. Cell Biol., 117:1263-75 (1992)). Human KHC N-terminal fragments have reportedly been expressed in E. coli. and purified (Fujiwara, et al., Biophys. J. 69:1563-8, 1995; Vale et al., Nature 380:451-3, 1996). Crystal structure of KHC motor domain has been reported (Kull et al., Nature 380:550-555, 1996). Motility activity of KHC has also been reported.
Another notable kinesin that has been identified is kinesin-like spindle protein (xe2x80x9cKSPxe2x80x9d), a member of the BimC kinesin family that is characterized by a conserved, globular motor domain at the amino terminus followed by a non-conserved, rod- like helical coiled-coil domain and a BimC box at the carboxyl terminus (Endow, Trends Biol. Sci. 16:221-225, 1991; Sanders et al., J. Cell Biol. 128:617-624, 1995). During mitosis, KSP associates with microtubules of the mitotic spindle. Microinjection of antibody directed against KSP into human cells prevents spindle pole separation during prometaphase, giving rise to monopolar spindles and causing mitotic arrest. KSP and related kinesins bundle antiparallel microtubules and slide them relative to one another, thus forcing the two spindle poles apart. KSP may also mediate in anaphase B spindle elongation and focusing of microtubules at the spindle pole.
Human KSP (also termed HsEg5) has been cloned and characterized (see, e.g., Blangy et al., Cell, 83:1159-69 (1995); Galgio et al., J. Cell Biol., 135:399-414, 1996; Whitehead et al., J. Cell Sci., 111:2551-2561, 1998; Kaiser, et al., J. Biol. Chem., 274:18925-31, 1999; GenBank accession numbers: X85137, NM 004523). Drosophila (Heck et al., J. Cell Biol., 123:665-79, 1993) and Xenopus (Le Guellec et al., Mol. Cell Biol., 11:3395-8, 1991) homologs of KSP have been reported. Drosophila KLP61F/KRP130 has reportedly been purified in native form (Cole, et al., J. Biol. Chem., 269:22913-22916, 1994), expressed in E. coli, (Barton, et al., Mol. Biol. Cell, 6:1563-74, 1995) and reported to have motility and ATPase activities (Cole, et al., supra; Barton, et al., supra). Xenopus Eg5 was expressed in E. coli and reported to possess motility activity (Sawin, et al., Nature, 359:540-3, 1992; Lockhart and Cross, Biochemistry, 35:2365-73, 1996; Crevel, et al, J. Mol. Biol., 273:160-170, 1997) and ATPase activity (Lockhart and Cross, supra; Crevel et al., supra).
Besides KSP, other members of the BimC family include BimC, CIN8, cut7, KIP1, KLP61F (Barton et al., Mol. Biol. Cell. 6:1563-1574, 1995; Cottingham and Hoyt, J. Cell Biol. 138:1041-1053, 1997; DeZwaan et al., J. Cell Biol. 138:1023-1040, 1997; Gaglio et al., J. Cell Biol. 135:399-414, 1996; Geiser et al., Mol. Biol. Cell 8:1035-1050, 1997; Heck et al., J. Cell Biol. 123:665-679, 1993; Hoyt et al., J. Cell Biol. 118:109-120, 1992; Hoyt et al., Genetics 135:35-44, 1993; Huyett et al., J. Cell Sci. 111:295-301, 1998; Miller et al., Mol. Biol. Cell 9:2051-2068, 1998; Roof et al., J. Cell Biol. 118:95-108, 1992; Sanders et al., J. Cell Biol. 137:417-431,1997; Sanders et al., Mol. Biol. Cell 8:1025-1033, 1997; Sanders et al., J. Cell Biol. 128:617-624, 1995; Sanders and Hoyt, Cell 70:451-458,1992; Sharp et al., J. Cell Biol. 144:125-138, 1999; Straight et al., J. Cell Biol. 143:687-694, 1998; Whitehead and Rattner, J. Cell Sci. 111:2551-2561, 1998; Wilson et al., J. Cell Sci. 110:451-464,1997).
The present invention provides a method for identifying an agent as a modulator of a kinesin. This method exploits the finding that loop 5 of the KSP motor domain is a binding site for a variety of test agents. The method comprises contacting a L5 loop-containing polypeptide with a test agent; and detecting a signal indicating binding between the test agent and the L5 loop.
According to a particularly preferred embodiment, the L5 loop-containing polypeptide is KSP or a fragment thereof, such as the motor domain of KSP. According to one aspect of the invention, the signal that is detected is fluorescence from a tryptophan residue within the L5 loop with a reduction of fluorescence following contacting the L5 loop with the test agent indicating binding of the test agent to the L5 loop.
Another aspect of the invention provides for a competitive binding assay, wherein the L5 loop containing polypeptide is contacted with a compound known to bind the L5 loop in addition to the test agent, and the signal indicates that the test agent competes with the compound for binding to the L5 loop.
According to another aspect of the invention, the method further comprises contacting the identified kinesin modulator with a kinesin molecule or a cell comprising a kinesin molecule, and determining an effect of the modulator on a bioactivity of said kinesin. In a particularly preferred embodiment, the bioactivity is mitosis, ATP hydrolysis, or bipolar spindle formation and the effect can be evidenced on cellular proliferation, cellular viability, motility, or morphology. In a more preferred embodiment, the modulator inhibits the bioactivity of KSP and produces a monopolar spindle phenotype.
The invention also provides a method of inhibiting cellular proliferation, comprising administering to a cell a composition comprising a kinesin modulator identified with the above method, thereby inhibiting proliferation of the cell.
A further aspect of the invention is drawn to a method for treating a disease in a patient, comprising administering to said patient a composition comprising an effective amount of kinesin modulator that specifically binds to the L5 loop region of a kinesin, and more preferably, of KSP, thereby treating said patient.
The invention also provides a fragment of a kinesin polypeptide comprising a L5 loop region of KSP.